1773, Iranian language spoken in Persia 3c.-10c., the language of the sacred books of the Zoroastrians, from Persian Pahlavi, from Old Persian Parthava "Parthia" (see Parthian).
Entries linking to pahlavi
Parthian n.
1520s, native or inhabitant of Parthia (ancient kingdom northeast of Persia in western Asia), from Old Persian Parthava- "Parthian," dialectal variant of the stem Parsa-, source of Persia. Partienes (plural) "Parthians" is attested from c. 1300.
As an adjective, "of or pertaining to the Parthians," by 1580s. The phrase Parthian shot is a figurative reference to their horsemen, who were expert at racing forward, turning, and shooting arrows backward at the moment of retreat. The exact phrase is attested by 1832; the image itself was in use long before (for example Parthian fight, 1630s).
Or, like the Parthian, I shall flying fight ["Cymbeline," Act I, Scene VII]